1. Field of the Invention
The present invention relates to a NAND flash memory for writing/erasing information by injecting charge into or removing charge from a floating gate or a charge storage layer.
2. Background Art
Nonvolatile semiconductor memories of the prior art include, for example, a floating-gate memory and a silicon-oxide-nitride-oxide-silicon (SONOS) memory (or a metal-oxide-nitride-oxide-silicon (MONOS) memory).
In these nonvolatile semiconductor memories, information (data) is stored by retaining charge (electrons or holes) in floating gates or charge storage layers which are isolated from surrounding parts by insulating films made of SiO2 and the like. A threshold voltage Vt of a memory cell transistor changes with an amount of retained charge and thus the information (data) is identified by sensing the threshold voltage Vt.
Information is written/erased, that is, charge is injected and removed by tunneling current generated by a high electric field from a Si substrate or a control gate, or by a hot carrier from the Si substrate (for example, see Japanese Patent Laid-Open No. 2003-173690).
In any injecting method, charge is repeatedly passed through the insulating film made of SiO2 and the like by repeating writing/erasing of information. When passing through the insulating film, the charge causes damage to the insulating film and generates a number of electron traps and hole traps.
The generated traps lead to various problems.